Recent high oil prices have increased interest in methods of producing oil at low cost. Accordingly, techniques for separating crude oil in massive amounts have been developed, and there is increasing interest in the oil sands industry. The term “oil sands” was originally used to refer to sand or sandstone containing crude oil and is now used to refer to all kinds of rock such as sedimentary rock that exist in oil reservoirs and contain crude oil. Oil production methods of extracting crude oil from oil sands are relatively new methods, as compared to the existing oil production methods of extracting crude oil from oil wells, and are expected to undergo further development to reduce production costs.
However, oil sands generally contain large amounts of impurities together with crude oil. Therefore, an impurity removing process is performed when extracting crude oil from oil sands. After mining oil sands, the oil sands are transferred a certain distance to separation equipment so as to extract crude oil from the oil sands, and then separation pipes are used to separate impurities and crude oil from the oil sands. In the separation pipes, crude oil and impurities (such as rock, gravel, and sand) are rotated using water to allow the crude oil floating on the water to be collected. Basically, such pipes are required to have a high degree of strength. In addition, such pipes are required to have impact resistance and abrasion resistance, because rock and gravel contained in the pipes impact the interior surfaces of pipes, and are required to have impact toughness to withstand low-temperature environments, for example, environments in which temperatures can fall to −29° C. Particularly, welding joints are strictly required to have such properties because welding joints are weaker than base metals. The physical properties of base metals may be adjusted through processes such as heat treatment processes, rolling processes, or controlled cooling processes so that the base metals may have the highest abrasion resistance and impact toughness obtainable from the compositions of the base metals. However, welding joints are mainly formed of welding materials and have internal structures similar to that formed by a casting process. Thus, it may be difficult to impart desired physical properties to welding joints.
Currently, pipes widely used for mining oil sands are API X65, X70, etc., and welding materials allowing for all-position welding are required to connect such pipes in the field.